1. Field of the Invention
This invention relates to porous amorphous aluminosilicate powders having uniform pore sizes and uniform particle size aluminosilicate aquasols. More specifically, this invention relates to porous amorphous aluminosilicate powders having a uniform pore size, precursor aluminosilicate aquasols with uniform particle size and their preparation by drying said aquasols to a powder without gelling.
2. Prior Art
Silica gels which are dried to porous silica powders are considered to be masses of spheres of SiO.sub.2 ranging from a few Angstroms up to several hundred Angstroms in diameter, which are aggregated together in a three-dimensional mass. Vysotskii ["Adsorption and Adsorbents" (No. 1), John Wiley & Sons, N.Y., 1973, p. 101] states that globular particles of silica form the skeleton of gels and that the cavities between the spherical particles constitute the pores in silica gels. This structure of silica gels is further described in detail by R. K. Iler, "The Colloid Chemistry of Silica and Silicates", Cornell University Press, Ithaca, N.Y., 1955, p. 129. The pore size and pore uniformity of silica is related to the particle size and particle uniformity.
U.S. Pat. No. 3,782,075 discloses a silica packing material for chromatographic columns having uniform-sized porous microspheres having substantially all of the microspheres in the range of from 0.5D to 1.5D where D is the average diameter.
It is known to react sodium silicate, sodium aluminate, and an acid, or sodium silicate, aluminum sulfate, and an acid to form a gel or precipitate of aluminosilicate directly. However, the prior art does not teach any method for controlling the ultimate size of aluminosilicate particles which eventually aggregate to form the gel structure or the preparation of the ultimate particles of uniform size. The control of the pore size distribution, namely the size distribution of spaces between these primary globules or aluminosilicate particles is likewise not known.
The difficulty of making aluminosilicate sol particles from which uniformly porous gels and powders can be formed is exemplified by Kontorovich, et al., J. of Colloid Chemistry, USSR (English translation), Vol. 35, p. 864, 1973 (Kolloyd, zhur, p. 935). Aluminosilicate particles, made simply by mixing sodium silicate, sulfuric acid, and aluminosulfate, showed a wide distribution of radii such that, for example, where the commonest particle radius was 20 A, a large fraction of the particles were also as large as 60 A radius. He further points out that even when the gels are aged for growth, the particles grow only to about 35 A, even after long exposure in water at 70.degree. C. He states definitely that the presence of aluminum in the globules hinders the increase in the size of the particles. This puts a limit in the A1 content for certain particle sizes.
The nonuniformity of pores of amorphous aluminosilicates is exemplified in U.S. Pat. No. 3,346,509 which discloses the preparation of silica-alumina compositions with a preponderance of the pore volume in pores of small radii. The pore radii are disclosed as ranging from above 200 A to less than 10 A with up to about 60% in the range of 10 A to 20 A.
U.S. Pat. No. 3,766,057 discloses an alumino-silica gel dried to a powder having a mean pore radius of 40 A to 100 A and 15% of the pore volume in a 10 A section with a wide distribution of particles in the adjacent particle sizes.
Making aluminosilicate sols of particles of 3 to 150 millimicrons in diameter which are uniform in chemical composition was described by G. B. Alexander in U.S. Pat. No. 2,974,108, issued Mar. 7, 1961. In U.S. Pat. No. 2,913,419, issued Nov. 17, 1959, Alexander discloses the preparation of gels and particles having a skin or outer surface of aluminosilicate composition. The gels have a coarse structure to permit coating with aluminosilicate without closing the pores in the gel. There is no disclosure of the need for uniform pores or for the preparation of uniform pore sizes or the control of the pH at a constant value between 9 and 12. Alexander's particles are used as filters while his gels are used as catalysts.
In porous catalyst powders, the uniformity in pore size is a definite advantage in affording specificity of reaction by avoiding side reactions and preventing the deposition of carbonaceous residues. Heretofore, it has not been possible to produce amorphous aluminosilicate catalysts with a uniform pore size.